This study is designed to test the hypothesis that colonial bats are an important interepidemic reservoir host of eastern equine encephalomyelitis (EEE) virus in southern New England and a mechanism for virus survival through the winter in a temperate climate. In the surveillance studies to date, EEE virus was not detected in blood and/or organ samples from 467 bats of four colonial species collected from 1973 through 1975 in Massachusetts and Connecticut or in nearly 1500 ectoparasites (ticks, bugs, mites, and fleas) from bats or roosts. Antibody was demonstrated in approximately 1% of the bats tested. Laboratory studies indicate that Aedes aegypti and Culiseta melanura mosquitoes will feed on nonhibernating bats and will transmit EEE virus from an avian host (day old cockerels) to bats. Preliminary results indicate that prolonged infections of EEE virus with recurrent viremias apparently dependent upon body temperature do occur in hibernating bats (Myotis, Eptesicus, Pipistrellus); infection may kill nonhibernating Eptesians fuscus in the laboratory although survivors do occur in nature as evidenced by antibody in feral bats collected in southern New England.